Initial support for annotations

[juruen]

So far the most common way to handle rM annotations by open source tools is to use the scripts that are available in https://github.com/reHackable/maxio/blob/master/tools/rM2svg

As we mentioned in #34, we would like to provide a similar functionality for rMapi. However, it'd be nice if we could avoid depending on multiple external tools and make it pure Go.

This PR, inspired by rM2svg and @peerdavid's work lays the foundations to do exactly that. We start by exporting annotations from Quick Sheets and notebooks.

This initial implementation doesn't support different brushes, tilts and pressure yet. But it's enough to render handwriting and simple sketches.

In this implementation I'm not using SVG and try to generate the PDF directly to simplify things.

/cc @peerdavid

[torwag]

Downloading an annotated PDF via rmapi geta (version 0.0.9), I get a segfault with the following backtrace:

geta testfile
downloading: [testfile]...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x5597dcd61124]

goroutine 1 [running]:
github.com/juruen/rmapi/annotations.(*PdfGenerator).Generate(0xc0004fba10, 0x0, 0x0)
	src/github.com/juruen/rmapi/annotations/pdf.go:62 +0x444
github.com/juruen/rmapi/shell.getACmd.func1(0xc00055c270)
	src/github.com/juruen/rmapi/shell/geta.go:54 +0x77a
github.com/abiosoft/ishell.(*Shell).handleCommand(0xc0001c60c0, 0xc0002a8520, 0x2, 0x2, 0xc0000e7c48, 0x20, 0xc000000180)
	pkg/mod/github.com/abiosoft/ishell@v0.0.0-20180125194311-42a9bbfa0017/ishell.go:248 +0x2fb
github.com/abiosoft/ishell.handleInput(0xc0001c60c0, 0xc0002a8520, 0x2, 0x2, 0xc0002aa101, 0xc0002ae000)
	pkg/mod/github.com/abiosoft/ishell@v0.0.0-20180125194311-42a9bbfa0017/ishell.go:202 +0x4f
github.com/abiosoft/ishell.(*Shell).run(0xc0001c60c0)
	pkg/mod/github.com/abiosoft/ishell@v0.0.0-20180125194311-42a9bbfa0017/ishell.go:181 +0x215
github.com/abiosoft/ishell.(*Shell).Run(0xc0001c60c0)
	pkg/mod/github.com/abiosoft/ishell@v0.0.0-20180125194311-42a9bbfa0017/ishell.go:97 +0x3b
github.com/juruen/rmapi/shell.RunShell(0xc00021a230, 0xc, 0xc0000e7ee8)
	src/github.com/juruen/rmapi/shell/shell.go:74 +0x951
main.run_shell(0xc00021a230)
	src/github.com/juruen/rmapi/main.go:14 +0x31
main.main()
	src/github.com/juruen/rmapi/main.go:38 +0x12e