moved isBuildslaveDir() to it's own module

Moved isBuildslaveDir() function from buildslave.scripts.runner module
to a new buildslave.scripts.base module. This way it is possible for to
move logic for checking that 'buildslave start' is run with valid
buildslave base directory inside buildslave.scripts.startup.start()
function.
changed error handling in buildslave.scripts.base.isBuildslaveDir()

 - Catch open/read errors while reading buildbot.tac, and print error
   message.
 - Print affected file and directory in error messages to ease
   troublshooting.
 - Moved all error messages inside isBuildslaveDir()
changed error handling in buildslave.scripts.base.isBuildmasterDir()

     - Catch open/read errors while reading buildbot.tac, and print error
       message.
     - Print affected file and directory in error messages to ease
       troublshooting.
     - Moved all error messages inside isBuildmasterDir()
Merge branch 'basedir-checks' of git://github.com/elmirjagudin/buildbot
py25 fixes
tryclient: support commas in property CLI

This adds a '--property' flag which contains single a property setting
and can be set multiple times (allowing for comments in property values).
Merge branch 'try_properties' of git://github.com/andrewjcg/buildbot