Inline Timer_A interrupt
Add tags file to .gitignore
Add low power sleep function for MSP430 LaunchPad

Add sleep() and sleepSeconds() functions.
On startup the init function will attempt to enable the 32KHz XTAL and
sets the ACLK to be sourced from the LFXT..
If this fails then the ACLK will fall back to either VLO or REFO depending
on the MSP430 flavor.
sleep()'s acuraccy is about +1 ms and sleepSeconds is about +250ms depending
on the MSP430. If sleep() is called with ms > 1000 then sleepSeconds will be called
for the whole seconds and sleep for the remainder milliseconds.

TODO: Adjust watchdog timer counter after sleep.
The watchdog timer counter is incremented at a different rate during sleep.
This causes micros to become inacurate after a long sleep period.
Merge branch 'master' of https://github.com/energia/EnergiaPrivate