Fix licence and add correct SPDX licence identifier

[tumbarumba]

• Moved LICENSE.txt to the more standard LICENCE, ensuring the text is the same as the canonical version at https://opensource.org/license/BSD-3-Clause
• Used the correct SPDX license identifier in the Bundle-License field in the manifest, as well as the pom
• Used the correct SPDX license identifier in the pom

Closes #264, Closes #355, Closes #399

[vlsi]

Thanks for looking into this.

I'm afraid the PR does not fix the core of #264.

See the second clause of the license:

2. Redistributions in binary form must reproduce the above copyright notice

Imagine somebody redistributes hamcreset.jar or includes it in their application. The license forces them to include the copyright notice, however, the release jars on Maven Central do not include the full license text. How would they know which exact license text (==copyright notice) matches every Hamcrest release?

See https://github.com/hamcrest/JavaHamcrest/pull/399/files#diff-49a96e7eea8a94af862798a45174e6ac43eb4f8b4bd40759b5da63ba31ec3ef7R75-R77

The link https://opensource.org/license/BSD-3-Clause points to a license template rather than a canonical text. Please make the link point to the license text, and and please include the license text as META-INF/LICENSE to make it easier for the consumers to comply with the license.

[tumbarumba]

Thanks for looking at this @vlsi. I'll make the suggested changes in the next few days

[tumbarumba]

Fix merged. @vlsi, can you please check?