[C] Update Maven Shade Plugin to version 2.0. Fixes BUKKIT-3213

##### The Issue:
With the maven-shade-plugin package relocation added in 8f12382e8efc8c39a919af9180dd884caf3720ff, the CraftBukkit classes include a larger constant pool full of both the relocated and un-relocated symbol names. Consequentially, the server jar file size has slightly increased, and automated analysis tools can become confused by the invalid symbols.

##### Justification for this PR:
* Reduces the size of the jar (slightly) to a size on par with pre-relocation CraftBukkit (about 13870568 to 13518181 bytes)
* Removes the invalid symbols fixing compatibility with certain binary class file analysis tools
* Fixes a regression introduced by the Version Safeguard
* Updates to the latest stable and recommended software version

##### PR Breakdown:
This PR simply updates the Maven project object model pom.xml to maven-shade-plugin 2.0 (instead of 1.4), which contains the fix: http://svn.apache.org/viewvc/maven/plugins/tags/maven-shade-plugin-2.0/src/main/java/org/apache/maven/plugins/shade/DefaultShader.java?revision=1380090&view=markup#L271

	        // We don't pass the ClassReader here. This forces the ClassWriter to rebuild the constant pool.
	        // Copying the original constant pool should be avoided because it would keep references
	        // to the original class names.

The .gitignore is also updated to ignore a new generated file from the updated plugin.

##### Testing Results and Materials:
Tested by applying the change, building CraftBukkit, and ensuring the server functioned as before in regular usage.

Built without and with the change, and confirmed the size reduction.

Extracted a class from the shaded jar, ran `javap` on it, before:

    Constant pool:
        #1 = Methodref          #10.#236      //  net/minecraft/server/ItemStack."<init>":(Lnet/minecraft/server/Block;I)V
        #2 = Fieldref           #237.#238     //  net/minecraft/server/Block.id:I
        #3 = Methodref          #10.#239      //  net/minecraft/server/ItemStack."<init>":(III)V
    etc.
    
after the change, the broken symbols are no longer present:
    
    Constant pool:
        #1 = Utf8               net/minecraft/server/v1_5_R2/ItemStack
        #2 = Class              #1            //  net/minecraft/server/v1_5_R2/ItemStack
        #3 = Utf8               java/lang/Object
    etc.


No plugin compatibility impact from this change. maven-shade-plugin is used only during the CraftBukkit build process, and is not (and cannot be) used as a library by Bukkit plugins.

maven-shade-plugin 2.0 is backwards-compatible with earlier versions, is widely used by numerous Java projects, and hasn't required any bug fixes for seven months.

This change has been live on several thousand servers for about three months. No problems reported.

##### Relevant PR(s): 

[B] https://github.com/Bukkit/Bukkit/pull/740 - Makes the same change in the Bukkit API project as well. This is not required and has a smaller benefit than the CraftBukkit change, but is submitted for consistency in keeping both projects up-to-date with the same version of maven-shade-plugin.
 
##### JIRA Ticket: 
BUKKIT-3213 - https://bukkit.atlassian.net/browse/BUKKIT-3213

##### Notes
I actually originally noticed this problem by running an analysis tool on the CraftBukkit jar, although I can't recall by now what tool it was. The shade comments mention compatibility with a bundling plugin which we don't use. But in either case, the size reduction is nice, not to mention the added compatibility and correctness.