fixed bugs preventing compile with Arduino 1.5.7

Hi Mikal, I just tried to use Flash in the Arduino 1.5.7 beta. A number of compiler errors prevents your example sketches from compiling. I assume that it's specific to the later versions of the Arduino toolchain. I noticed on [your website that Liss posted a number of fixes](http://arduiniana.org/libraries/flash/) for items that were previously compiler warnings. I implemented those fixes in this fork, and now Flash compiles and seems to operate correctly in 1.5.7.  I take no credit for identifying these changes, but I thought I'd make them available to you in github in case that's easier for you to evaluate them.