mirroring calabash-ios console command

* introduced a CALABASH_IRBRC variable

1. set IRBRC to CALABASH_IRBRC if it exists
2. if ./.irbrc is present, set IRBRC to this local .irbrc
3. otherwise IRBRC is et to the calabash-android gem irbrc file

* launching irb with ruby `exec` rather than starting with a system
  call.  This is preferrable because `exec` swaps out the current
  process with `irb` rather than running `irb` in the context of
  current process.
using single quotes vs. double quotes (style change)
console cmd prints the path the to the irb init file
Merge branch 'master' into fix/console-rvm-irbrc
Merge branch 'fix/console-rvm-irbrc'
Add ability to specify main_activity with enviroment variable MAIN_ACTIVITY.
Do not invoke methods to find env options if they have been specified in start_test_server_in_background
Fail if unable to uninstall specified app
Look for zipalign in build-tools in not found in tools