Split out SLF4J and log4j version props for Maven

This centralizes the SLF4J and log4j versions to a single place in the
Maven build system: the toplevel pom.xml.

Note that the Maven build system currently uses a different version of
log4j than the Ant build system does: 1.2.14 rather than 1.2.15. (For
those curious: the reason is that SLF4J 1.5.10 declares 1.2.14 as the
version of log4j with which it works.) In order to minimize changes,
this commit does not change that fact. But the next one will upgrade and
standardize the chosen versions across both build systems.
Upgrade SLF4J to 1.7.2 across the board

This also upgrades log4j to 1.2.17, which is the corresponding version
for that version of SLF4J.

The reason for the upgrade is to improve the behavior of SLF4J in case
there is no logger implementation binding present. Specifically,
starting with SLF4J 1.6, SLF4J defaults to a no-op logger when no
binding is present, rather than throwing NoClassDefFoundError This is
much nicer for downstream consumers of Bio-Formats, since they do not
need to worry about always specifying an SLF4J binding as a dependency
in addition to Bio-Formats.