Add poetry build info into pyproject.toml

This doesn't appear to work at the moment, as when building the wheel, I
get the following output:

    Building django-schema-graph (1.1.0)
     - Building sdist
     - Built django-schema-graph-1.1.0.tar.gz

    [IndexError]
    list index out of range

When I remove `schema_graph/static/schema_graph/main.js` from
`.gitignore`, the build succeeds... but I want that file to be ignored.

Related to #11

.gitignore

@@ -10,3 +10,4 @@ build
 dist
 node_modules
 schema_graph/static/schema_graph/main.js
+poetry.lock

pyproject.toml

@@ -0,0 +1,20 @@
+[tool.poetry]
+name = "django-schema-graph"
+version = "1.1.0"
+description = "An interactive graph of your Django model structure."
+authors = ["Charlie Denton <charlie@meshy.co.uk>"]
+license = "MIT"
+packages = [
+    { include = "schema_graph" },
+    { include = "schema_graph/static/schema_graph/main.js" },
+]
+
+[tool.poetry.dependencies]
+python = "~2.7 || ^3.5"
+attrs = ""
+
+[tool.poetry.dev-dependencies]
+
+[build-system]
+requires = ["poetry>=0.12"]
+build-backend = "poetry.masonry.api"