Improve performance of getTokens (Closes #189)

Closes #189 (putting this into a title of the pull request doesn't work for me half the time, don't know why)

I re-wrote getTokens to improve performance. I decided not to add tokens directly to ast, and instead I just populate them into hashtable with startLocation as a key.

Performance increase is pretty good. Running eslint with default rules on jshint file before the changes:
Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 41
Milliseconds      : 917
Ticks             : 419175419
TotalDays         : 0.000485156734953704
TotalHours        : 0.0116437616388889
TotalMinutes      : 0.698625698333333
TotalSeconds      : 41.9175419
TotalMilliseconds : 41917.5419

after the changes:

Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 1
Milliseconds      : 968
Ticks             : 19688548
TotalDays         : 2.27876712962963E-05
TotalHours        : 0.000546904111111111
TotalMinutes      : 0.0328142466666667
TotalSeconds      : 1.9688548
TotalMilliseconds : 1968.8548