Reconsider e.printStackTrace and add logging

[grobmeier]

At this line:
https://github.com/wildbit/postmark-java/blob/master/src/main/java/com/wildbit/java/postmark/Postmark.java#L51

the stack trace is written to console. In clustered environments this message could get lost, as the console output is disabled. Logging messages are usually directed using a logging framework.

One dependency is already using slf4j:

[INFO] +- eu.medsea.mimeutil:mime-util:jar:2.1.3:compile
[INFO] |  +- org.slf4j:slf4j-api:jar:1.5.6:compile
[INFO] |  +- org.slf4j:slf4j-log4j12:jar:1.5.6:runtime
[INFO] |  \- log4j:log4j:jar:1.2.14:runtime

Unfortunately it's an older version. Another option would be to use JUL directly. However, usually slf4j is being used as it's compatible with all major frameworks and is also offering basic logging for those who don't want to include them.

[ibalosh]

agreed, will consider it in next release

tx!

[ibalosh]

should be resolved in 1.1.4 version