set InstallActivity as singleInstance, there should ever only be one

There is no reason to have multiple InstallActivity instances running, it
can take a fair amount of CPU/memory to load the install.log text file into
a TextView.  Often, singleInstance behaves weirdly, but I think it should
make sense here because this is a one time, long running process.

refs #137 https://github.com/guardianproject/lildebi/issues/137
write install log directly to file; broadcast updates to InstallActivity

This commit should drastically improve the performance of the install
process.  The old process was really horribly inefficient, it reposted the
entire contents of the log in order to update a single line.  This commit
also makes it so you can follow the log file in realtime using `tail -f` or
whatever.

closes #137 https://github.com/guardianproject/lildebi/issues/137