Fixed: Check for alternative rst2pseudoxml binary.

On some systems, the corresponding binary is named "rst2pseudoxml"
instead of "rst2pseudoxml.py".
add a function to get the active checker names for a filetype
Merge pull request #492 from tobyS/fix-rst-alternative-checker-binary

Fixed: Check for alternative rst2pseudoxml binary.
rst: extract a temp var to a query
add an faq RE setting which checkers to use
first round of updates to the doc

* remove the changelog, credits and (outdated) doc on writing syntax
  checkers
* doc the `syntastic#makeprg#build()` options
* doc how to select which syntax checkers to use
Added explicit call of escript

Added explicit call of escript, to run check, bypassing shell (actual for Windows)
Merge pull request #496 from mixvin/patch-1

Added explicit call of escript
twig: use syntastic#makeprg#build
add an faq about option names changing and remove changelog from readme
readme: fix a couple of typos
vala: remove the option to disable it

This can be done with:

`let g:syntastic_mode_map={'passive_filetypes': 'vala'}` or by setting
`let g:syntastic_vala_checkers=[]`

Closes #502.