Makefile: Add suport for multiple locale builds

This is my humble attempt to support building xpi with multiple locales.

#### Improvements on current Makefile:

* When building help files first the English help files get copied into the
  locale folder, then the localized help files are copied over them. This way
  only translated help file will have to live in the locale subdirectories, any
  files not translated yet would not go 'stale'.

* 'make xpi' now includes in the built xpi help files from all available
  locales. Available locale names are existing vimperator|muttator/locale and
  common/locale subdirectory names.

* 'make info' now shows all available locales we have translated help files for

Did some test to see how the vimperator xpi file size woudl change with more and more locales added:
    * one locale (en_US): 579KB 
    * two locals: 665KB
    * three locales: 735KB
    * four locales: 805KB
    * five locales: 875KB
Looks like every extra locale added increases the size of the compressed xpi by about 70KB.


#### Translation workflow

The suggested workflow for translating the help documentation to another language could be as follows:
* translator forks the vimperator project to translate help files into $LANG
* creates common/locale/$LANG and vimperator/locale/$LANG directories
* decides which help file to translate first
* copies the original English version of the file to its $LANG directory
* translates file
* builds xpi with 'make xpi'
* tests that everything works OK
* commits changes
* grabs another file to translate or creates pull request