First attempt at improved section movements

Adresses issue #76

I think this should work well (at least it does in my own test case).  However,
I was not able to split it up in the way it has been done for similar mappings.
That is, I tried to use <Plug> and to define the function in motion.vim, but
for some reason it never worked.

Also, the [] and ][ mappings could probably still be defined in a better way.
Improved section based movements
[] and ][ now works when used more than once in a row

Ref. issue #76
Simplified [[, [], ][, and ]] mappings

Rewrote the search function with inspiration from
http://learnvimscriptthehardway.stevelosh.com/chapters/51.html
Ignore commented lines in section motions
Optimization for insert mode
* Use smart trick to prevent foldexpr function being called too often

See: http://permalink.gmane.org/gmane.editors.vim.devel/14100

Addresses issue #77
Also fold section[...] and similar

Enable folding also for sections and chapters that use the optional [] clause,
and set appropriate fold text.
Smarter initial fold level
Also check if g:LatexBox_Folding == 1

Addresses issue #81.
Optimized folding, ref #82
fixed build directory check for output file. fixes #83
Merge pull request #84 from dynosure/build_dir_fix

Fixe build directory check for output file.

Ref. #83 and #84
Don't disable %-mappings when matchparen is off

Ref. #58:  Thanks to Tertium for noticing this.
Minor bug fix
Minor fix to fold text
Fixed autocommand to kill latexmk
* Ref. issue #87

The previous autocommand fails when the tex-buffers are deleted before one
exits vim.  This update changes the autocommand for kill_all_latexmk() to
a buffer specific autocommand for the event BufUnload.

Also add a simple check in kill_all_latexmk to prevent error messages if the
event is triggered after the buffer has been unloaded, which seems to happen
sometimes.
Fixed a minor error in foldtext functions
Two minor updates

1. Added simple enhancement to allow a comment to set a fold caption (#89)
2. Minor fix to frame fold captions
Removed stupid echom (sorry about that)
original file