New plugin, line spacing and rework of cell plugin

First of all: this is a big merge request and I understand if I need to divide in small merges.
For a personal project, I had to adapt several things on jsPDF. What I changed is:
- a new plugin to put the total number of pages (like FPDF does).
   it is quite intrusive and I had to change the internals of jsPDF to implement
- Slightly larger line spacing
   also, it allows an user to configure the line space
- Big rewrite of cell plugin
   which allow for multiline cells (automatically if the content is bigger than the cell width), format headers, allow custom headers (to put a title or logo) and right aligned cells.

Please review and tell me which changes are interesting for you to merge.