strip InvocationTargetException from stack traces

We really just care about the cause.
add lineSeparator property to NewlineAtEndOfFile checkstyle module to avoid spurious errors on Windows.
fix issue #47 - run enforce goal during Eclipse builds.
provide more detailed explanatory message with IllegalArgumentException
thrown when conflicting Producer methods are detected.
Add OttoBus base interface, and have Bus implement it.
Creating a base interface allows instances of classes that act like a Bus but aren't a Bus (e.g. ScopedBus) to be passed around as a standard interface. To minimize disruption to existing code, I left the name of the Bus class alone and named the interface OttoBus.
fixed comment block indentation
restore blank lines in copyright/license comment header.
Merge pull request #55 from adennie/base-interface

Add OttoBus base interface, and have Bus implement it.
Merge pull request #48 from adennie/issue47

fix issue #47 - run enforce goal during Eclipse builds.
Merge pull request #46 from adennie/checkstyle-line-separator

add lineSeparator property to NewlineAtEndOfFile checkstyle module to av...
Merge branch 'better-conflicting-producer-exception' of https://github.com/adennie/otto into adennie-better-conflicting-producer-exception
Package scope the variable directly.
Merge branch 'master' of github.com:square/otto
Merge branch 'unpack_ites' of https://github.com/loganj/otto into loganj-unpack_ites
Formatting.