Lib now passes all tests

Added back original startWrite function for when users want to use
manual methods. The library uses startFastWrite for its internal
functions. Provides compatibility with prev. code while still using the
new methods for everything else.
txStandby() function modified completely

txStandBy() now either works in two modes. The main mode will block
until success or fail. Using txStandBy(1) will block and retry forever
until succesful. Using a while loop with the function is no longer
required. See the updated class reference at
http://tmrh20.github.io/RF24/class_r_f24.html