Switch to standard logging framework

[jn1z]

Currently the code uses a combination of System.out.println(), ex.printStackTrace(), and writing to a StringBuilder.

Thankfully these uses are now mostly isolated to a few calls, but this could be improved with the following benefits

• the purposes of efficiency, i.e. not holding the entire log in memory
• better output, i.e. not outputting full stack traces to the user
• maintainability, i.e. not passing around print flags in the code

Recommended is SLF4J + logback, but whatever's not too complex.